There’s no doubt that great strides have been made in air pollution. Awareness, stricter legislation and improved technology have all contributed to improved air, land and water conditions. In fact, the nation’s air quality is much cleaner than it was in some of the worst-affected areas, according to a recent report by the American Lung Association. Air emissions that contribute to pollutants have fallen since 1970 thanks to the Clean Air Act.

Despite the improvements, 4 in 10 Americans live where pollution levels are often dangerous to breathe. Since the ALA began studying particle pollution, almost all of the most polluted cities have consistently remained among the worst. Pittsburgh has been one of the 10 most polluted cities since 2004. The cities of Bakersfield and Merced, in California, are the most polluted cities in the U.S. this year and have been among the 25 most polluted since at least 2004. They are among many California cities, including Los Angeles, that have struggled with pollution for some time.

The ALA’s 2013 “State of the Air” report measures cities based on low-lying ozone pollution, as well as both short- and long-term particle pollution. These particles, just 1/30th the diameter of a human hair, are capable of getting past our bodily defenses and cause physical harm, particularly to those who already suffer from pulmonary diseases, the very young and the elderly. The report measures both the total accumulated particle pollution over the course of a year, as well as the number of days that the air pollution hit unhealthily high levels.

Several of the most polluted cities are in California's San Joaquin Valley. Extremely heavy traffic worsens the valley's air quality, accounting for as much as 89% of pollution there.

Janice Nolen, the ALA’s assistant vice president for national policy, also noted that the San Joaquin Valley has had major agricultural growth in recent years. Agriculture creates pollution in a variety of ways, from the actual tending of the crops to the vehicles that bring supplies in and food out. A review of Census data shows that agriculture, fishing and forestry represent more than 10% of the economy in many of the highly polluted metro areas, compared with roughly 2% of all jobs nationwide.

The Los Angeles port is also a major source of pollution, with the diesel exhaust from the oceangoing ships generating significant particle pollution. While part of the high pollution levels in these cities comes from local sources, Nolen noted that much of it also comes from being downwind from cities that are even larger producers of pollution: “California has historically had the biggest challenge in its cities. Part of that is the sources, and part of it is the geography — pollution that might blow somewhere else gets captured in the valleys.”

A review of the pollution levels in these California cities shows that while they remain among the worst in the country for both particle and ozone pollution, they have been improving markedly in recent years. Los Angeles, for example, has reduced its number of unhealthy particle pollution days by more than 50% since 2000.

This improvement reflects the state’s efforts to deal with the problem, Nolen explained. “California has really been, in many ways, leading the nation, because they recognized they had the worst problems.” When the Clean Air Act was signed into law in 1970 the state was given additional resources and authority to be more aggressive, said Nolan.

Based on average long-term particle pollution figures collected by the ALA between 2009 and 2011, 24/7 Wall St. identified the 10 most polluted cities in the country. Merced’s year-round particle pollution ranked as the worst in the nation, in a tie with the Bakersfield-Delano metro area. We also reviewed ozone pollution and short-term particle pollution, which the ALA measured as the number of days between 2009 and 2011 where pollution levels were deemed unhealthy. For each of these metropolitan areas, the ALA noted the population and the number of people in the area with health problems that high pollution can exacerbate, such as asthma and cardiovascular disease.

The greater Cincinnati area is one of the most polluted areas in the country outside of California. Only 13 other metropolitan areas averaged more days with dangerously high ozone levels. However, to the city’s credit, the number of days with high ozone levels in the area fell by more than half between the 1996-1998 measurement period to the 2009-2011 one. Much of the greater Cincinnati area is in Ohio, which was the second worst state for air pollution in the nation, according to a 2012 study from the National Resources Defense Council.

In the last year, the El Centro metro area had 49 days where the ozone level was unhealthy for sensitive populations. In addition, the ALA gave the metropolitan area a failing grade for particle pollution. El Centro’s population of about 177,000 people includes many residents who suffer from conditions that can be exacerbated by pollution. Much of El Centro’s air pollution has to do with vehicle emissions, and the city has made efforts to address the problem by recently converting several city streets into pedestrian malls. Fortunately, the number high ozone days has fallen from an average of more than 80 annually in 1996-1998 to less than 20 in 2009-2011.

Since the ALA began measuring particle pollution in 2004, Pittsburgh has been among the 10 cities with the highest levels of particle pollution. In the most recent study, it was seventh worst overall for short-term particle pollution, and eighth worst for long-term pollution. Nolen explained that one of the major sources of continuing particle pollution in the area is the U.S. Steel Plant. Pittsburgh is also downwind from many coal-fired power plants in the midwest. This also has a major impact on the region’s particle pollution levels. As a major city, Nolen added, heavy traffic also contributes to high pollution levels. An estimated 28% of the region’s 2.45 million residents suffer from cardiovascular disease, with high pollution levels putting them at risk of further exacerbating their condition.

The Visalia metro area registered an average of 234 days a year of ozone air quality that was considered unhealthy for sensitive populations between 2009-2011. In addition, in 19 days the ozone air quality was considered unhealthy for everyone. According to the ALA, particle pollution in Kings County, the only county in the Visalia area , has doubled in the last six years. Out of roughly 450,000 residents 36,000 have asthma, and the poor air quality may be causing them significant problems. In addition, more than 86,000 people in the area suffer from cardiovascular disease.

Modesto is one of several cities among the nation’s worst for air pollution located within the Central Valley in California. The valley has high pollution levels due to both its unique geography and its massive agricultural industry. The Modesto Convention & Visitors Bureau reported that agriculture represented over one-third of all jobs in the area and was among the nation’s leading producers of milk, almonds and poultry. The metro area received failing grades for the number of high ozone days it had, the number of high particle pollution days, and the annual average levels of particle pollution. Additionally, while the number of high ozone days has declined in recent years, the number of days where particle pollution is especially high has risen dramatically — there were 16.2 more such days in the most recent measurement than there were in the 2000-2002 period.

The greater Los Angeles area is home to more than 18 million people, more than twice the combined population of all nine other metro areas on this list. With many, if not most, residents driving regularly — the area is among the worst for traffic congestion — air quality in the greater Los Angeles area has suffered. Almost 4 million residents of the greater metro area suffer from cardiovascular disease — the symptoms of which can be made-worse by exposure to high levels of air pollution. No other area in the country had more high ozone days than Los Angeles, which was the only city that averaged over 100 such days per year between 2009 and 2011. According to the Los Angeles Daily News, the South Coast Air Quality Management District, which oversees air quality in the greater Los Angeles area, is working to reduce emissions from older diesel vehicles and is pitching an emission-free system for moving cargo containers at local ports.

Although the Hanford metropolitan area is less than 1% the size of the Los Angeles area, the pollution problem is even bigger. From 2009 to 2011, Hanford area had 98 days on average per year where ozone pollution was considered unhealthy for sensitive populations, and seven additional days where it was considered unhealthy altogether. Similarly, there were 89 days where particle pollution was considered unhealthy for sensitive populations, and an additional 21 days where it was considered downright unhealthy. The Hanford area is in the San Joaquin Valley, which generally suffers from very poor air quality. It is also next to Bakersfield and Hanford, the worst and third-worst metros in terms of air pollution. Of the area’s population of nearly 154,000 people, more than 12,000 have asthma, the symptoms of which are exacerbated by high pollution levels.

In addition to being ranked as the third-worst place in America for year-round particle pollution in 2009-2011, the greater Fresno area ranked among the worst for the number of individual days where particle pollution was especially high, as well as the number of days with elevated ozone levels. Just over 1 million people live in the greater Fresno-Madera area, and more than 224,000 of them suffer from some sort of cardiovascular disease, the symptoms of which can be aggravated by pollution. Some of the area’s pollution troubles may be due to the high numbers of commuters. The area’s population rose 15.7% between 2000 and 2010.

Merced’s year-round particle pollution ranked as the worst in the nation, in a tie with the Bakersfield-Delano metro area. While the metro area did not rank as poorly for the number of days with especially unhealthy particle pollution, the figure has been on the rise in recent years. In response to the figures released by the ALA, Merced mayor Stan Thurston defended the city to the Merced Sun Star, claiming that much of the pollution actually comes from the San Francisco Bay Area. According to the San Joaquin Valley Air Pollution Control District, an estimated 27% of emissions measured in Merced county originated in either the San Francisco Bay Area or Sacramento area.

Along with Merced, no city had more of a problem with pollution than the Bakersfield metropolitan area. The ozone air quality was considered unhealthy for sensitive populations an average of 199 days per year from 2009 to 2011, while it was considered flat-out unhealthy in another 24. Fortunately, the average number of unhealthy days in the last two years dropped 44% since 1996-1998, despite the fact that population grew 41% in just the last decade. The area has resorted to unusually high-tech solutions to combat pollution. Earlier this year, NASA flew aircrafts over the area equipped with scientific tools to experiment with ways to better measure air quality.